Wait, how do y'all know that jaeyi's brother? did the author announced it?

Probably because his silhouette showed when they had mentioned his brother previously and the last names?

Well, he introduced himself as a famous competitive swimmer Keon Jiseong. In an earlier chapter Jaeyi's collegemates found out about his brother and that he used to do Judo in school.

*Kwon Jiseong